Question

Which one of the following statements is totally wrong about the occurrence of a notochord, while the other three are correct?
A. It is present only in the larval tail in Ascidians
B. It is replaced by a vertebral column in adult frog
C. It is absent throughout life in humans from the very beginning
D. It is present throughout life in Amphioxus

Answer 1

Notochord forms the skull and the vertebral column. It is present in all the chordates due to which they are known so. Moreover, the notochord forms the skull and the backbone in all mammals including humans.

Complete step by step answer: Let us look at all the statements one by one.
Statement A: It (Notochord) is present only in the larval tail in Ascidians - Ascidia is a member of the Urochordata of the group Protochordata. Protochordate is also known as Invertebrata or Acraniata. This is because Notochord is present but it cannot form the vertebral column or the cranium or skull. Urochordata is also known as Tunicata because of the presence of the tunicin cover. Urochordates are characterized by the presence of a Notochord found in the tail of the larva. This notochord dissolves later on due to which well-developed larva cannot form a well-developed adult.
Statement B: It (Notochord) is replaced by a vertebral column in adult frog - Frog belongs to the Amphibia class of phylum Chordata. It belongs to the group Vertebrata or Craniata. These organisms are characterized by the presence of a notochord that changes into the backbone and skull.
Statement C: It (Notochord) is absent throughout life in humans from the very beginning - Humans belong to the class Mammalia of phylum Chordata. It belongs to the group Vertebrata or Craniata. These organisms are characterized by the presence of a notochord that changes into the backbone and skull.
Statement D: It is present throughout life in Amphioxus - Amphioxus is a member of the Cephalochordata of the group Protochordata. Protochordate is also known as Invertebrata or Acraniata. This is because Notochord is present but it cannot form the vertebral column or the cranium or skull. Cephalochordates are characterized by the presence of a Notochord in the head and the thorax region which is persistent throughout the life of these animals.
Thus, based on the above information we can conclude that the statement that notochord is absent throughout life in humans from the very beginning is wrong.
Hence, the correct answer is option (C).

Note: Notochord is differ from the nerve cord. The nerve chord is that cord which forms the brain and the spinal cord whereas the notochord forms the skull and the backbone or the vertebral column. A nerve cord is a dorsal tubular and hollow structure originating from the ectoderm. On the other hand, the notochord is the dorsal tubular and solid structure originating from the mesoderm.